1. Field of the Invention
The present invention relates to a light emitting device using a nitride-based III-V group compound semiconductor or a light emitting device using a fluorescent substance.
2. Description of the Related Art
There has been attempted acquisition of white light based on emission of a plurality of types of lights having different peak wavelengths by using light emitting diodes (LEDs) formed of GaN and its mixed crystal in place of a conventional white fluorescent lamp (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-352098). In this publication, an Si-doped GaN photoexcitation emission layer is excited by blue light generated in a multiple quantum well (MQW) light emission layer of a light emitting diode to generate yellow light, and the fact that these lights have a complementary color relationship is utilized, thereby obtaining white light. However, a red color cannot be generated in the light emitting device described in the above publication, color rendering of light is hence degrated, and it is hard to say that the light emitting device can sufficiently substitute for the conventional white fluorescent lamp.
In recent years, a white lamp obtained by determining an LED of GaN and its mixed crystal as an excitation source and combining the LED with a fluorescent substance has been utilized for illumination. However, there is a problem that efficiency and color rendering of the white lamp are inferior to those of a conventional fluorescent lamp using a mercury discharge tube. That is because a fluorescent substance used in this white lamp has a wide spectral width and low light emission efficiency. In particular, a most common cause is low light emitting efficiency of a red fluorescent substance.
In the conventional white fluorescent lamp, an oxide or the like in which a trivalent rare earth element Eu is added (e.g., Y2O3: Eu3+) is used as the red fluorescent substance. This oxide having Eu added therein demonstrates red light emission with a very narrow wavelength distribution in the vicinity of 620 nm by an inner core transition inherent to the trivalent rare earth element Eu. Since the wavelength distribution is narrow, there is no loss due to light emission in a long-wavelength region where a luminosity factor is low, which greatly contributes to an improvement in efficiency and color rendering.
This inner core transition of the trivalent rare earth element Eu atom is a forbidden transition, and efficiency is drastically increased when excitation occurs by light having a short wavelength which is not greater than 350 nm. Therefore, when the LED formed of GaN and its mixed crystal is utilized, since a light emission wavelength of GaN is 365 nm, excitation must be performed with a wavelength shorter than this wavelength. However, in case of shortening the light emission wavelength, it can be considered that sufficient efficiency cannot be obtained due to, e.g., light absorption by GaN formed on a sapphire substrate or the like.
As described above, in case of exciting the red fluorescent substance by using the light emitting diode formed of a semiconductor such as GaN and its mixed crystal or the like, light emission efficiency of the red fluorescent substance is low. It can be considered that sufficient efficiency cannot be obtained due to light absorption or the like by the semiconductor such as GaN even if the light emission wavelength is shortened in order to improve the light emission efficiency. Therefore, the red fluorescent substance cannot be allowed to efficiently emit light by using the light emitting diode formed of a semiconductor such as GaN and its mixed crystal or the like. Further, it is difficult to realize a light emitting device having high efficiency and high color rendering by combining the red fluorescent substance with any other visible light fluorescent substance.
Therefore, there has been demanded realization of a light emitting device which allows the red fluorescent substance to efficiently emit light or a light emitting device using a light emitting diode and a fluorescent substance which is superior in efficiency and color rendering.